I was soo looking forward to building the ExStatA hybrid until I've read the last few pages of this thread. I even started collecting parts, and was just about to place a $220 mouser order.

spritzer: a few pages back you mention the bias supply is finicky. How does the problem manifest itself?

If the caps measure fine is there any need to replace them, even after 20 years?

you cannot measure if a cap is good or bad with a capacity meter. If they dry up, they might still have the same capacity but their esr rises, making them pretty useless.

I'd go for a set of nice new low-esr caps on any device that has been in heavy use for a couple of years (say 5-15) and tends to get a lot of heat - even if it still performs well. Caps are cheap and maintaining those might save you a lot of trouble..

Does anyone have a spare headband for sale for an SR-3 or SR5? My SR5 headband is cracked and cannot be repaired with superglue or anything short of custom-made parts.

Parts for any of the pre-Lambda models are very thin on the ground and Stax shows no stock. Your best bet would be with any of the larger Stax distributors which have been in buisness since the 80's (Germany being very big on the SR-5's back in the day) or watching ebay.

As for running the bias a bit hot, the driver can in theory easily handle 500V bias but Stax have always been rather conservative with bias voltage in context of the D/S gap.

Speaking of caps. I replaced the main filter caps and the low voltage filter caps in my Stax SRA-series amplifier about 20 to 25 years ago, so theoretically it's overdue for a new set (some say every 10 years!). The amp has been performing very well, however. I have an LED on it that takes 6 seconds to go out on power off so there's a decent amount of reserve capacity. I just measured the current (pun not intended) caps in situ and they all measure at least 30% higher than the specified value. They were all good-model electrolytics. My question is : If the caps measure fine is there any need to replace them, even after 20 years?

I just replace everything but then again I'm not normal. That said, the caps in my old SRM-T2 measured like new but replacing them with fresh units cured the offset issues I was having and raised the voltage rails up to spec again.

If I remember correctly the old SRA units were all AC-coupled so you might want to look into new output caps as well.

spritzer: a few pages back you mention the bias supply is finicky. How does the problem manifest itself?

My board haven't arrived yet to no testing as of yet but people were experiencing odd things like the volume of the system drifting as if the bias supply wasn't working correctly. There were also some issues with the first supplies (as in they never worked) but I converted mine to the voltage doubler with a voltage divider at the end setup from the start so I never had any issues.

As for the exstata, I've been cleaning up the layout of the new boards to try and make them smaller for the last couple of days and I've gotten them down to 195x122mm which isn't bad for something which has the same number of heatsinks as the KGSS two channel board. A simple PSU coming up too and then I'll order some boards.

My board haven't arrived yet to no testing as of yet but people were experiencing odd things like the volume of the system drifting as if the bias supply wasn't working correctly. There were also some issues with the first supplies (as in they never worked) but I converted mine to the voltage doubler with a voltage divider at the end setup from the start so I never had any issues.

Ah, I see. So something like the bias supply from the blue hawaii would work here, right?

As for the exstata, I've been cleaning up the layout of the new boards to try and make them smaller for the last couple of days and I've gotten them down to 195x122mm which isn't bad for something which has the same number of heatsinks as the KGSS two channel board. A simple PSU coming up too and then I'll order some boards.

Nice! I assume it is the SS version with KG's fixes? I want better THD too. >.>

BTW, about this:

The heatsinks need to be bigger, and thermal cutouts have to be added to the heatsinks to shut

the thing down. And half the power going to the shunt devices needs to be taken up by power resistors

bolted to the chassis.

I'm having a hard time trying to understand which resistors in the schematic this is referring to. Or does it mean that we should add a series of power resistors before the shunt devices? If so, which devices in the schematics are the shunt devices? /noob questions

Ah, I see. So something like the bias supply from the blue hawaii would work here, right?

Yup, that's exactly what I used.

Nice! I assume it is the SS version with KG's fixes? I want better THD too. >.>

Yup, but I'll only do the SS version. For the tube version it would just make more sense to build the KGST...

I can't seem to find boards or parts list for the KGST at the moment, other than a few pictures of the prototype and estimated cost at around $2K (!!!) from headamp. So regardless of the exstata's high THD, I think it should be a good start into diy electrostatic amps for me. I guess it shouldn't be to bad once I:

1) replace the bias supply

2) add thermal breakers to heatsinks on the PS

3) Mod the boards with KG's fixes for the hybrid (if possible, likely not...)

4) and most importantly, rethink about my budget for this build

I think at this point I'm just going to turn around and sell my extata boards and wait for the KGSSHV. Great first stax-related DIY project, eh?

I think at this point I'm just going to turn around and sell my extata boards and wait for the KGSSHV. Great first stax-related DIY project, eh?

Well, there's always that option too.

I can't seem to find boards or parts list for the KGST at the moment, other than a few pictures of the prototype and estimated cost at around $2K (!!!) from headamp. So regardless of the exstata's high THD, I think it should be a good start into diy electrostatic amps for me. I guess it shouldn't be to bad once I:

1) replace the bias supply

2) add thermal breakers to heatsinks on the PS

3) Mod the boards with KG's fixes for the hybrid (if possible, likely not...)

4) and most importantly, rethink about my budget for this build

There is 1 KGST in the wild and I have it. No boards as I'm using a modified board from another amp design and a Blue Hawaii PSU.

As for modding the stock Exstata board, it might be a bit hard...

you cannot measure if a cap is good or bad with a capacity meter. If they dry up, they might still have the same capacity but their esr rises, making them pretty useless.

I'd go for a set of nice new low-esr caps on any device that has been in heavy use for a couple of years (say 5-15) and tends to get a lot of heat - even if it still performs well. Caps are cheap and maintaining those might save you a lot of trouble..

Thanks! So the only way to tell if there's a filter cap problem is by listening... and change the caps and see if the listening improves? (There's no hum; the regulators and I assume the caps are doing their static job). I assume that bass performance is the place to listen for an improvement or degradation. I wish there were an objective way to assess bad or old filter caps.

Bob

There is, but you need special equipment capable of measuring the characteristic curve of electronic components for it. If you want to try listening, then go for the highs - because high esr <> slow cap dis-/recharging (lowpass) -> possibly rolled off treble. But I don't assume that there will really be an audible difference...

Edited August 2, 201015 yr by Cynric

One way to check the caps is to measure the HT rails. If they are on spec then the caps work properly.

One way to check the caps is to measure the HT rails. If they are on spec then the caps work properly.

I'm not so sure of that. You're looking at an unregulated voltage and it could be off by any amount depending on the line voltage and other variables. Perhaps looking at the raw ripple on an oscilloscope and/or measuring the residual AC RMS level on the DC line. But at the Prodigy forum I learned that a cap may have good capacitance but its ESR could have been shot and this may not show up by looking at the ripple, or would it? Bottom line is various authorities I've consulted seem to indicate that the only sure way of knowing if an old filter cap is bad if the DC and ripple measure good is to replace it and then listen! (how primitive)

BK

Well you could check the AC off the transformer and then do the math but yeah, the most simple way is just to replace them.

Btw. Which of the SRA's is this? I might have the schematic if you need it. There was also a restoration thread here on the SRA-12S a while back...

Well you could check the AC off the transformer and then do the math but yeah, the most simple way is just to replace them.

Btw. Which of the SRA's is this? I might have the schematic if you need it. There was also a restoration thread here on the SRA-12S a while back...

Thanks. I already have the service manual. It "was" an SRA-12S which I proudly bought new in 1972! Now that it's been stripped; it's just a Stax amplifier, no preamp or booster section. I've changed the volume control to my own hand-built stepped volume control, a few other goodies. I'll have a look for the restoration thread, it will be interesting.

WPI connector fail. With everything sourced dirt cheap from China, how hard can it be to get teflon Stax jacks for next to nothing?

Especially since Woo already has their own Stax jacks that they use on their other amps. I assume this is a transformer box?

They have switched to WPI on everything now which makes no sense to me and yeah, that's the transformer box. I compared it briefly at CJ against my Lundahl box and even the the much lower ratio of the Lundahl's they were much smoother to my ears and the bass more controlled even off the WA5.

[clueless n00b question]I'm guessing the sockets are WPI connectors? Does WPI stand for anything in particular?[/clueless n00b question]

CMIIW, WPI is the company selling stax pro connector

Yup WPI is the company that makes them but these are the exact same molds as have been sold by Amphenol since at least the 1960's. My Heathkit tube checker has a socket like this and so did the old Koss ESP7,9 and 10

I still have four of those suckers and thought they'd be useless, but maybe not. Now I can say I'm using the same jacks Woo use. What I don't understand is why WPI/Amphenol made it so each of the metal pieces is loose. What is the point of that?

Crap tolerances maybe. The oldest Stax connectors weren't so great either but the newer Sato parts are better.

^^Have you tried replacing the pins in those connectors with the taiwanese tube pins that you posted in the diy forums?